Carbon fiber features low specific gravity, high tensile strength, high modulus, high electric conductivity, and high thermal conductivity and has the advantage of a soft fiber that can be woven. After being processed by high temperature, the carbon fiber is a fiber material with a carbon content of more than 90%. The carbon fibers can be classified into different types according to different methods including the classification by raw material, feature, processing temperature and shape. When the carbon fibers are classified according to the raw material, there are six types, respectively: a cellulose-fiber rayon series, a lignin series; a polyacrylonitrile (PAN) series; a pitch series, a phenol resin series, and a gas carbon fiber series. When the carbon fibers are classified according to the features such as mechanical strength and the modulus, there are five types of carbon fibers as listed below:
(1) Ultra-high module fiber with a tensile modulus greater than 600 GPa;
(2) High module fiber with a tensile modulus with a range of 350˜600 GPa;
(3) Mid module fiber with a tensile modulus with a range of 280˜350 GPa;
(4) Standard module fiber with a tensile modulus with a range of 200˜280 GPa; and
(5) Low module fiber with a tensile modulus smaller than 200 GPa.
Wherein, the tensile modulus is defined as the level of easiness of deforming an object by a force. The higher the modulus, the more difficult is the object to be deformed. A special carbon fiber with a high modulus is used extensively as an enhanced composite material for construction, aviation, and military. Therefore, it is an important subject for related manufacturers to manufacture a fiber with higher modulus.
In FIG. 1, the carbonization temperature rises during the manufacture of carbon fibers to increase the fiber strength, but the fiber strength will remain at a saturation point after the fiber graphitization reaches an ultra high temperature over 2,000° C., and then the temperature fiber strength starts dropping and the modulus will increase instead. In FIG. 2, the modulus decreases with the fiber strength when the graphitization process temperature increases (up to 2,000˜2,500° C.).
In the future trend, high-strength high-modulus carbon fibers are produced with a low cost. If the graphitization temperature of the raw material of a general high-strength carbon fiber is increased, the module may be increased, but a significant drop of strength will result. Therefore, it is a main subject for manufacturers to improve the level of graphitization stacking and the performance of modulus while maintaining the high strength of the high-strength carbon fibers.
Among the conventional carbon fiber manufacturing technologies, different heating graphitization methods are used in high temperature graphitization and graphitization process, and some of the common conventional thermoelectric finance type graphitization processes as disclosed in Japan Pat. No. JP200780742, R.O.C. Pat. Nos. 561207, 200902783 and 279471 focus on improving the manufacturing processes that adopt a conventional thermoelectric furnace. In other words, a high temperature furnace is used for heating in the carbonization process, and different heat exchange methods are used to transmit heat energy from the outside to the inside while heating the external cavity, insulation facility, protective atmosphere and fiber. However, the drawbacks reside on low heat conduction, difficult insulation, taking too much time to heat to the desired temperature since the temperature rising speed is affected by the heat conduction effect, and the graphitization and carbonization process takes over 1˜10 hours, so that the high modulus carbon fiber manufacturing process generally fails to take the high strength into consideration. In general, the heating method not only takes a long time, but also wastes unnecessary energy. In addition, a large quantity of insulation devices is required for a good heat insulation system to prevent heat loss of the high temperature electric furnace. The required graphitization furnace and the increased cost make the mass production difficult and increase the cost of the carbon fibers.